High levels of S. mutans in dental plaque are associated with dental caries, while high levels of S. sanguis are associated with noncariogenic plaque. An inverse relationship between the number of these organisms recovered from plaque samples has been noted. Factors suggested to be responsible for this relationship include selective adhesion to the tooth or preexisting plaque surface, production of bacterial inhibitory products by one or both species, as well as selection promoted by environmental conditions such as pH, pO2, and nutrient availability. The working hypothesis for the proposed studies is that interactions between S. mutans and S. sanguis as well as environmental factors determine the composition of dental plaque and thus determine the state of health or disease on the tooth surface. Additionally, it is postulated that the relative importance of selective factors will differ for smooth and fissured tooth surfaces. These hypotheses will be addressed using continuous flow cultures because bacteria grown in this manner are physiologically more similar to bacteria grown in vivo in terms of their growth rates, enzyme profiles and metabolic activity. Bacterial growth parameters and interactions will be characterized in a continuous flow system and compared to results obtained from static cultures. The system will then be modified to study the initial bacterial attachment and interbacterial adhesion involved in plaque formation. Effluents of S. mutans and S. sanguis cultures will flow over hydroxyapatite and enamel smooth and fissured surfaces and the resulting plaque will be characterized. Environmental conditions will be individually altered to study their effects on this process. By studying the basic interactions between plaque bacteria and their environment, factors which control and possibly select for a protective normal flora can be better understood, and potentially implemented in the development of biologically based therapeutic regimens.